Single-photon emission computed tomography (SPECT) imaging has provided the practicing clinician with a method of studying brain function in patients with dementia. A large and growing number of papers report the experiences of a number of laboratories in the use of this technique in the evaluation of demented patients. Studies from several laboratories comparing patients with Alzheimer's disease to control subjects report sensitivity and specificity of SPECT perfusion imaging to be in the 80% vicinity. In addition, a number of studies suggest that the dementias that show the greatest similarities in perfusion patterns to Alzheimer's disease are multi-infarct dementia and dementia associated with Parkinson's disease. Although considerable data exists to guide the physician, a rigorous scientific approach to studying patients in a prospective, unselected clinical sample, with autopsy confirmation of the diagnosis, is needed to define clearly the utility of the technique in diagnosing dementias.